Many integrated circuits now in use are fabricated in what is called CMOS (complimentary metal oxide semiconductor) technology, which forms both PMOS and NMOS transistors on a silicon substrate. One of the main objectives of integrated circuit technology is to minimize transistor size. Transistors are typically described in terms of their minimum feature dimension.
Current technology provides a minimum feature size of 0.35 micron or less. The minimum feature size, which is also referred to as a "line width", refers to the minimum width of a transistor feature such as the gate width, or the separation between source and drain diffusion regions. Typically, 0.35 micron technology is used to form CMOS transistors having a gate oxide thickness of 70 .ANG. (angstroms). A 0.18 micron technology is used to form CMOS transistors having a gate oxide thickness of around 35-40 .ANG.. A 0.15 micron technology is used to form CMOS transistors having a gate oxide thickness of around 25-30 .ANG.. The gate "oxide", is typically an oxide dielectric layer that is interposed between the conducting gate electrode, which is typically a polycrystalline silicon structure formed overlying the principal surface of a substrate in which the integrated circuit if formed, and the underlying substrate which typically is the channel portion of the transistor extending between the source and the drain regions. Transistors formed using the 0.35 micron technology typically operate at a voltage of 3.3 volts. Transistors formed using the 0.18 micron technology typically operate at a voltage of 1.8 volts. Greater voltages are likely to destroy the transistor by rupturing the gate oxide. Sub 0.35 micron processes provide for two different transistor families having different gate oxide thicknesses. A first transistor family has a thin gate oxide and a second transistor family has a thick gate oxide.
In the field of data storage, there are two main types of storage elements. The first type of storage element is a volatile storage element such as typically used in DRAM (dynamic random access memory) or SRAM (static random access memory) in which the information stored in a particular storage element is lost when power is removed from the circuit.
The second type of storage element is a non-volatile storage element in which the information stored in the storage element is preserved even if power is removed from the circuit. Typically, the types of elements used to provide non-volatile storage are substantially different from those used in ordinary logic circuitry or in volatile storage, thereby requiring different fabrication techniques.
It has heretofore not been possible to include non-volatile storage on an integrated circuit chip formed exclusively using standard CMOS processes.